During a combustion process of an internal combustion engine, fuel is oxidized and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds.
Automobile exhaust systems typically include a catalytic converter that reduces the levels of CO, HC, and NOx in the exhaust gas by chemically converting these gasses into carbon dioxide, nitrogen, and water. The catalytic converter has a property known as Oxygen Storage Capacity (OSC). OSC refers to an ability of the catalytic converter to store excess oxygen when operating under lean conditions and to release oxygen when operating under rich conditions.
A vehicle may operate in a fuel cut-off mode when the vehicle is operating in an overrun condition. Overrun conditions may occur when decelerating, coasting down a hill, or when a manual transmission clutch is engaged. During the fuel cut-off mode, the engine continues to pump gases resulting in delivery of oxygen to the catalytic converter. The oxygen is stored in the catalytic converter until a maximum oxygen storage capacity is attained. When the catalyst reaches the maximum oxygen storage capacity, NOx breakthrough may occur, reducing the ability of the catalytic converter to convert NOx emissions. Thus, it is desirable to reduce the amount of oxygen stored in the catalyst when the vehicle is operating in the fuel cut-off mode.
In one method, extra fuel is delivered to the engine and into the catalyst after fuel cut-off has occurred. The oxygen is removed through catalytic action with additional HC and CO provided by the extra fuel. However, this method requires additional fuel and therefore fuel efficiency is reduced.